Babies can smile at birth, or even sooner  -- ultrasounds have actually spotted upside-down frowns on babies in the last trimester of pregnancy. What's unclear is why they do it so early on. "Some research shows that they smile most often as they're falling asleep and waking up," says Alan Fogel, Ph.D., professor of psychology at the University of Utah in Salt Lake City and author of Infancy: Infant, Family and Society. "Those times are probably inherently pleasurable for babies, and their good feelings seem to create a smile."

It is true that social smiling  -- when a baby breaks into a gummy grin in response to something outside herself, like the sight of your face or the start of a favorite routine  -- doesn't begin until 2 months. By the time your baby is 3 to 4 months old she'll be able to pull off broader smiles that lift her cheekbones and may include laughing. You may grow addicted to making her beam at you, but follow her cues: If she smiles and then turns away, she could be overstimulated. Give her a break, let her recharge, and remember there are years of smiles yet to come!

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